Some great tips on how to save your tabs when you restarting Firefox:

1. Use the Restart Firefox extension.
2. Set Firefox to save all windows and tabs, every time.
3. Set Tab Mix Plus to save all windows and tabs (if you are using the Tab Mix Plus sessions instead of the native Firefox one.)

http://www.howtogeek.com/howto/internet/firefox/quick-tip-save-windows-and-tabs-when-restarting-firefox/